The invention is based on a priority application EP08291252.8 which is hereby incorporated by reference.
The invention relates to a handover method in a base station of a mobile communication system, to a base station and to a computer program product.
In a cellular network, a cell typically has several neighboring cells. The handover procedure allows the continuation of a call when a user equipment crosses the border of one cell to another. The basic concept of handover is that when a user terminal moves from the coverage area of one cell to another, a new connection to the target cell is set up and the connection to the old cell may be released. During the handover, data interruption may occur caused by delays in signaling message, or cause by problems during the signaling with the candidate base station and the source base station.
In general, when a user equipment moves from one cell to another in a cellular radio system and the handover is performed, the handover may be described as a hard handover or a soft handover. During the hard handover, the old connection is broken before a new one is set up. Therefore, handover margins are used in order to diminish the problem of data interruption. However, the base station connection cannot be utilized in such a case due to the handover margin and the delay of the handover procedure. During the soft handover, updating margins are also employed.
Further, a pilot signal is used to identify a base station and to form an active set in a mobile communication system. The pilot signal is continuously transmitted by each base station to its coverage area. The user equipment can identify the base stations on the basis of the pilot signal, since the signals differ from one another. The user equipments continuously measure pilot signals and maintains a measurement list of the base stations and the corresponding signal quality levels of the pilot signals that are situated near the user equipment and that are possible candidates for handover or connection establishment.
The base stations on the measurement list form a group of candidates that may become members of the active set. User equipments may monitor the pilot signals of only those base stations that are on the measurement list. When a user equipment moves, the measurement list is updated as the need arises. The updating process is performed according to the measurements of the user equipment based on the strength of the pilot signal. That means that if a pilot signal transmitted by a base station is received with adequate strength, it is added to the measurement list.